On May 23, 2005, at 9:16 AM, Manuel Bouyer wrote:

> In if_xennet, data received from the interface is passed to the kernel
> in a mbuf with external storage (MEXTADD). In the external storage
> free  routine (xennet_rx_mbuf_free()), there is a comment saying
> that that there may be some MMU operation pending for this buffer
> (in Xen, some MMU operations are delayed via a MMU update queue),
> and the MMU update queue must be flushed. This cause problems,
> and I'm wondering if there can ever be MMU operations queued for
> these pages, once they have been MEXTADD()'d to a mbuf and passed  
> to the
> network stack. If would make my life much easier if the anserw was  
> "no" :)

I don't think there are any cases where something takes additional  
mappings of an mbuf external buffer for the inbound case.  If  
something wants to reference that buffer, then it needs to actually  
take an mbuf external storage reference to it.  By the time your  
xennet mbuf_free routine is called, you are guaranteed that there are  
no other references to that buffer.

-- thorpej